1. Field of the Invention
The subject of the present invention is an amplifier for standby system, operating permanently and able to be "woken up" by electronic signals of origin external to the amplifier, for example: alternating or continuous magnetic field, modulated or unmodulated electromagnetic field, modulated or continuous infrared field, etc.
2. Description of the Related Art
When the amplifier uses a battery for supply, it is necessary periodically to change the latter. Now, in certain devices, the battery cannot be replaced, or not without great difficulty. It is therefore necessary, in this case, to provide an amplifier of high gain, but of the lowest possible consumption. Such devices are used in antitheft devices for automobiles, electronic transaction devices for urban or cross-country automobile traffic toll facilities, pedestrian access control; "hands-free" toll devices in public or private means of transport.
A device of this type generally requires the detection of several signals of different kinds, in particular:
frequency or amplitude modulated high-frequency HF signals; PA1 infrared signals; and PA1 mechanical signals after passing through a transducer. PA1 the current consumed in order to carry out electronic transactions is negligible and would lead to a theoretical lifetime of around 142 years for a battery with a capacity of 100 mAh; PA1 the lifetime of the battery is, in practice, constrained solely by two elements, namely: its leakage current and the value of the current consumed permanently by the amplifier which it supplies. PA1 a first stage furnished with a first input able to receive a first signal, and with an output for delivering said first signal after amplification and/or demodulation, PA1 at least one second stage linked in series to the first stage and furnished with a first input able to receive the signal from the output of the first stage, and with an output for delivering the signal after amplification and/or demodulation, and PA1 a low-voltage supply linked in series with the stages, and capable of supplying the stages with DC current. PA1 each stage is supplied with the same DC current, this making it possible to minimize, on the one hand, the current consumed by the detector amplifier as a function of the characteristics of each of its constituents, and on the other hand, the low-voltage supply voltage as a function of the gain of each amplification stage and of the minimized current, and PA1 at least the second stage comprises at least one second input able to receive a second signal to be amplified and/or demodulated. PA1 a first stage including: PA1 a second stage including: PA1 a third stage including:
These signals are in general low level. Their processing therefore requires high-gain amplification, this in general necessitating a large current. It is therefore essential to minimize the consumption of an amplifier on permanent standby, if it is desired that it operate for as long as possible given the capacity of the supply battery.
Such an amplifier must be able to amplify signals of different types which may exhibit different impedances. The conventional solution consists in providing an amplifier for each input, the outputs of the amps being linked to an active or passive summing unit which processes the signals in order to deliver a correct output signal on the single output of the amplifier. However, given that in such a device each stage is individually linked to the supply, this requires the use of a battery of high capacity, and hence large volume, since the consumption is multiplied by the number of stages.
Taking a battery with a capacity of 100 mAh, it can yield a maximum current of 3.80 .mu.A for three years, namely substantially 26,300 hours. If, now, the amplifier consumes a current i=3 .mu.A, and if it is "woken up" 24 times per day in order to carry out an electronic transaction during which it requires an instantaneous consumption of 3 mA for 100 ms, the mean current, to carry out these transactions, is then determined to be: I mean=3.times.0.1/3600=0.08 .mu.A.
From these two examples it is possible to deduce that:
For modern lithium batteries, the manufacturers frequently indicate a lifetime of five to ten years, which, in the case of a capacity of 100 mAh, corresponds to a leakage current of between 1.15 .mu.A and 2.3 .mu.A. Thus, for a battery with a capacity of 100 mAh and three years of consumption, by assuming that the leakage current is just 1.5 .mu.A, the permanent current available to activate the standby device is just 2.3 .mu.A.
To try to limit the consumption of an amplification circuit, those skilled in the art have proposed placing the amplification and power supply stages in series.
The German Patent DE-A 27 34945 (R. BOSCH) and American Patent US-A 5 041 797 (D. K. BELCHER) describe such amplifiers.
These comprise:
The major drawback of this type of amplifier lies in the fact that it is able merely to amplify a single signal from the input of a first stage. Stage (n) serves merely to amplify the signal delivered by stage (n-1) directly preceding it.
If it is wished to process several different signals, several amplifiers of this type must therefore be juxtaposed, this again requiring several power supplies.